By virtue of excellent magnetic properties, Nd—Fe—B permanent magnets find an ever increasing range of application. The recent challenge to the environmental problem has expanded the application range of these magnets from household electric appliances to industrial equipment, electric automobiles and wind power generators. It is required to further improve the performance of Nd—Fe—B permanent magnets.
Indexes for the performance of magnets include remanence (or residual magnetic flux density) and coercive force. An increase in the remanence of Nd—Fe—B permanent magnets can be achieved by increasing the volume factor of Nd2Fe14B compound and improving the crystal orientation. To this end, a number of modifications have been made on the process. For increasing coercive force, there are known different approaches including grain refinement, the use of alloy compositions with greater Nd contents, and the addition of effective elements. The currently most common approach is to use alloy compositions having Dy or Tb substituted for part of Nd. Substituting these elements for Nd in the Nd2Fe14B compound increases both the anisotropic magnetic field and the coercive force of the compound. The substitution with Dy or Tb, on the other hand, reduces the saturation magnetic polarization of the compound. Therefore, as long as the above approach is taken to increase coercive force, a loss of remanence is unavoidable. Since Tb and Dy are expensive metals, it is desired to minimize their addition amount.
In Nd—Fe—B permanent magnets, the coercive force is given by the magnitude of an external magnetic field which creates nuclei of reverse magnetic domains at grain boundaries. Formation of nuclei of reverse magnetic domains is largely dictated by the structure of the grain boundary in such a manner that any disorder of grain structure in proximity to the boundary invites a disturbance of magnetic structure, helping form reverse magnetic domains. It is generally believed that a magnetic structure extending from the grain boundary to a depth of about 5 nm contributes to an increase of coercive force (see K. D. Durst and H. Kronmuller, “THE COERCIVE FIELD OF SINTERED AND MELT-SPUN NdFeB MAGNETS,” Journal of Magnetism and Magnetic Materials, 68 (1987), 63-75). For providing both a high coercive force and a high remanence, it is ideal that the concentration of Dy and Tb be higher in proximity to grain boundaries than within crystal grains.
An effective approach for achieving such a morphology is, as disclosed in WO 06/43348 by the present applicant, by disposing a powder containing one or more of oxides, fluorides, and oxyfluorides of rare earth elements on a surface of a sintered magnet body and heat treating the magnet body at a temperature below the sintering temperature in vacuum or an inert gas. This approach is referred to as “grain boundary diffusion process,” hereinafter. With this process, Dy or Tb is incorporated into the sintered magnet body from the rare earth compound present on the sintered magnet body surface and diffused into the magnet body along grain boundaries. It is believed that diffusion of Dy or Tb only in proximity to grain boundaries facilitates to increase the coercive force. This causes a little or no loss of remanence because the substitution amount of Dy or Tb is very small relative to the overall crystal grains.
In general, the grain boundary phase of Nd—Fe—B permanent magnet includes a Nd-rich phase, a Nd oxide phase, and a B-rich phase. Among these, the Nd-rich phase becomes a liquid phase during the heat treatment, and Dy or Tb is dissolved in this liquid phase and diffused into the interior, which enables diffusion into a deep portion of the magnet having a depth of millimeter order, despite the relatively low temperature which is below the sintering temperature.